It is sometimes desirable to be able to automatically monitor the level of both opaque and clear liquids without contacting the liquids. For example, in a blood oxygenator the damgerous consequences of allowing the blood level to fall too low in the arterial blood reservoir--infusion of gas into the patient--makes it highly desirable to monitor blood level. Optical monitors provide a sterile (no fluid contact) means to do so, but correct operation of the level monitor in an oxygenator is best confirmed before blood is introduced by monitoring the level of a clear saline priming solution.
Oishi et al. U.S. Pat. No. 3,636,360 shows monitoring the liquid level of a pressure vessel by passing a light beam from a bulb centrally through a transparent tube communicating with and positioned vertically alongside the vessel and relying on the difference in relative refractivity between a wall-to-air interface and a wall-to-liquid interface to cause the light beam to emerge from the tube along different paths so as to only activate a phototransistor when air occupies the monitored portion of the tube. It is mentioned that opaque liquid filling the tube will prevent light from reaching the sensor and that the bulb and phototransistor, shown supported in separate projector and receiver housings mounted on a flat plate, could be adjusted vertically. In one embodiment the transparent tube is triangular in cross section.
Bird U.S. Pat. No. 3,065,354 shows a liquid level monitor contained within a sealed tube extending downward into a reservoir. Collimated light is passed through a prism one surface of which is exposed to the reservoir. Depending on whether liquid or gas occupies the monitored level, light is refracted at the exposed surface along one of two paths, then reflected by a mirror facing the prism, and finally viewed through a window by a photoconductive cell, the window being positioned along only one of the two light paths.
Heule U.S. Pat. No. 3,851,181 shows a blood level monitor for an oxygenator. The monitor has a plurality of sensors spaced vertically along one transparent wall. Each sensor uses a pair of LED's emitting flashing 1000 Hertz infrared light to illuminate the reservoir and a single phototransistor to sense light reflected by the blood.
Hass U.S. Pat. No. 2,882,520 teaches a level detection means based on light absorption for detecting the height of a column of mercury.